Metal cleaning



llnited States Patent 3,330,769 Patented July 11, 1967 3,330,769 METALCLEANING Michael Golben, Maplewood, and John W. Pearson, MendotaHeights, Minn, assignors to Minnesota Mining and Manufacturing Company,St. Paul., Minn., a corporation of Delaware No Drawing. Filed Jan. 23,1964, Ser. No. 339,606 5 Claims. (Cl. 252136) This invention relates tothe cleaning of metal parts prior to the applicaton of an overcoatingmaterial. It particularly relates to the cleaning of metal parts priorto the application of a metal by mechanical plating.

In recent years mechanical plating has replaced electroplating in manyinstallations, especially where manufacturers are concerned wthrelatively small parts. In mechanical plating the parts are firstcleaned to remove metal oxide, oil, and dirt, etched, and then placed ina tumbling barrel in the presence of finely divided metal powder, apromoter chemical, and, generally, impact media. As the tumbling barrelis rotated or otherwise agitated, the metal powder is squashed againstthe surface of the parts, forming a continuous coat. Even though themechanical plat ing process is convenient and reduces the floor spacerequired, equipment to clean the surface before plating has stilloccupied a disproportionate amount of the manufacturers facilities.

For many years prior to my invention the conventional way to clean ametal surface has been a multi-step batch operation. Parts to be cleanedwere first degreased to remove oil and organic contaminants, either byexposing them to trichloroethylene vapor or by immersing them in a tankof caustic solution. The degreased parts were then removed, rinsed in asecond tank, removed, subjected to a descaling, or oxide-removing, acidpickle treatment by immersing them in a third tank, removed and againrinsed, often in a fourth tank. The tanks occupy considerable floorspace, thereby inherently increasing the cost of the process. Thiscleaning operation also requires maintaining a continual check on thestrength and contamination of the treating baths, and the open tanksrepresent a serious danger to personnel. Further, where high-carbonsteel, or inadequately degreased, parts are pickled, the inorganicpickling acids often leave a carbon smut on the metal surface, making itextremely difficult to obtain uniform coverage of an overcoatingmaterial. In addition, acid pickling tends to trap hydrogen in the metallattice, making the metal more brittle (i.e., hydrogen embrittlement),frequently resulting in breakage of a cleaned and subsequentlyelectroplated part.

The present invention is believed to provide for the first time aunitary one-shot process for cleaning metal parts to be plated, therebyeffecting a great economy in both cleaning time and requisite plantspace. The process is convenient, eliminating the need for the constantcareful maintenance of several treating baths and the transfer of partsfrom one bath to another. It is simple and inexpensive, producingconsistent and predictable results. In accordance with the invention,the metal parts to be cleaned are placed in a container (which may bethe same barrel used in mechanical plating) and a relatively smallamount of aqueous cleaning solution, and, preferably, impact media areadded. The parts and solution are agitated until the parts are cleaned,after which the cleaning solution is flushed away, leaving the cleaned,degreased, descaled, desmutted etched parts ready for plating. Bydiscarding the used cleaning solutionwhich is not economically feasiblein a dip cleaning method-contamination is avoided, and quality controlis facilitated. For cleaning parts by this unique one-shot method, a newand useful metal cleaning composition has been devised, comprising acombination of ingredients which facilitates the practice of theprocess. This combination quickly and simultaneously effects the diverseand heretofore separate functions of degreasing, descaling, desmutting,and etching, while essentially eliminating the problem of hydrogenembrittlement. The cleaning composition comprises a water-solublemixture of metal oxide-removing acid, hydrogen embrittlement inhibitor,and surfactant having the combined properties of metal wetting, oxidescale and dirt detergency, and oil emulsification.

The scale-removing acid used in the cleaning solution may be eithermineral or organic, the former being relatively inexpensive and thelatter being both easier to control and less corrosive at roomtemperature. Descaling and etching 10,000 grams of typical metal partsrequires enough acid to supply between 1 and 5 grams of availablehydrogen ion. Either monobasic or polybasic acids may be employed, butfor purposes of this invention a hydrogen ion is considered to beavailable if it has an dissociation constant of at least 10- In order tominimize hydrogen embrittlement, I also prefer to include a hydrogenembrittlement inhibitor. Whether or not an inhibitor works with theparticular acid used can be determined empirically by known procedures,of the type described in the specific example following.

Generally speaking, the required range of properties possessed by thesurfactant can best be obtained by blending surfactants which aresimilar in ionic nature but different in surface-active characteristics.One recognized way to classify surface active agents is to establishtheir hydrophilelipophile balance (HLB), a trial and error procedure forwhich has been discussed at length by William C. Griffin; see, e.g., theJournal of the Society of Cosmetic Chemists, vol. I (1949), pages311-326 and vol. V (1954), pages 249256. A somewhat more sophisticatedtechnique is to establish (the ratio of the volume of oil to volume ofwater at the inversion point between oil-in-water and water-in-oilemulsions) and, from known values, graphically determine the HLB; seepages 378-3 83 of Interfacial Phenomena, J. T. Davies and E. K. Rideal(1961) Academic Press, New York, London.

Surfactants are molecules having a hydrophilic portion and ahydrophobic, or lipophilic, portion, so that in another sense the HLBnumber is related to the ratio between hydrophilic groups and lipophilicgroups in the molecule. Those surfactants in which the lipophilic groupsare dominant tend to be oil-soluble and hence are extremely effective inwetting surfaces which may be contaminated with organic materials, e.g.,oils; such surfactants tend to have an HLB value of 5 or less.Surfactants in which the hydrophilic and lipophilic groups are roughlybalanced have an HLB value in the proximate range of 12 to 17 and tendto function as detergents if their molecular weight is relatively lowand as emulsifiers if their molecular Weight is relatively high.Surfactants in which the lipophilic groups are dominant have and HLBvalue of more than 17 and function effectively as dispersants forcarbonaceous or other materials.

In the preferred practice of this invention a mixture of surfactants isemployed. One surfactanta wetting agent which loosens oil and dirt andaids the acid in uniformly contacting the metal oxide scale-has an HLBvalue in the range of 1 to 5. Another surfactant-a detergent which aidsin suspending dirt and scalehas an HLB value in the range of 12 to 17,and a third surfactantan emulsifier which emulsifies residual oils-hasan HLB value in the range of 12 to 17. Desirably, a fourth surfactant-adispersant which prevents particle agglomeration and redeposition andwhich has an HLB value greater than 17-is also present.

As indicated in the preceding paragraph, emulsifying ability is arequisite for the composition of this invention. Oftentimes parts to becleaned are supplied in very oily condition; for example, 10,000 gramsof metal articles may include 150 grams or more of oil. As a convenientway to predict the emulsifying ability of a surfactant, the qb andtherefrom the HLB, is determined with an emulsifying machinesubstantially like that described on pages 378 et seq. of the Davies andRideal book, previously mentioned. In this machine oil 1 and water arepumped through flow gauges from separate reservoirs at controlled ratesand fed between two polymethyl methacrylate shearing plates. A stream ofemulsion impinges on a polymethyl methacrylate window; a continuouswhite film appears if the oil phase is continuous, and white dropsappear if the water phase is continuous. A precise way of determiningthe inversion point involves measuring the electrical conductivity ofthe emulsion, which decreases sharply as the change from water-in-oil tooil-inwater occurs, and conversely increases sharply as the change fromoil-in-water to water-in-oil occurs. Flow rates are adjusted toestablish the oil:water ratio, at phase inversion.

As a further aid to understanding the invention, a presently preferredcomposition and its method of preparation will now be described. Allparts are by weight unless otherwise noted.

EXAMPLE Hydrogen embrittlement inhibitor To 50.2 parts of de-ionizedwater is added 32.0 parts of a water-soluble high molecular weightpolyoxyethylene glycol. The charge is heated to 120 F. and maintained atthat temperature until all the polyoxyethylene glycol has been dissolvedand a uniform solution has resulted. The temperature is then lowered to75 F at which time the following ingredients are added: 4.8 parts ofconcentrated hydrochloric acid, 6.5 parts of o-toluidine, and 6.5 partsof 37% formaldehyde, and the reaction kettle sealed. When the exothermicreaction has ended, the charge is heated briefly to 120 F., cooled to 85F. and drained from the container. This composition is hereinafterreferred to as Composition A.

Preparation of metal cleaning composition To a stainless steel kettle ischarged 84.4 parts of 70% glycolic acid, 1.8 parts of Pluronic L-64, and1.8 parts of Pluronic L-lOl. Agitation is commenced and 1.8 parts ofPluronic P-85 and 1.8 parts of Pluronic P- 104, both normally solidmaterials which have been previously heated to render them fluid, areadded. Mixing is continued until the composition is uniform, at whichtime 4.4 parts of Composition A is slowly added. Mixing is continueduntil the solution is completely uniform. The final composition has aspecific gravity of 1.23.

The Pluronics referred to in the preceding paragraph are heatandacid-stable, nontoxic non-ionic surfactants made by adding propyleneoxide to propylene glycol in sufiicient amount to form a hydrophobicnucleus, and thereafter graft polymerizing ethylene oxide on each end ofthe polyoxypropylene nucleus to form two hydrophilic polyoxyethylenechains. The greater the molecular weight of the nucleus for a givenpolyoxyethylene:polyoxypropylene ratio, the greater the hydrophobic (orlipophilic) nature of the surfactant; conversely, the greater the lengthof the polyoxyethylene chains for a givenpolyoxyethylene:polyoxypropylene ratio, the more hydrophilic thesurfactant. Pluronic L-64 is a liquid which has a polyoxypropylenenucleus having an average molecular weight of about 1,750,polyoxyethylene making up approximately 40% of the total weight of themolecule; this surfactant has an HLB value of 15 and is useful as acarbon soil remover. Pluronic L-101 is a liquid having apolyoxypropylene nucleus with an average molecular weight of Mineralspirits, or ligroin, i.e., the fourth fraction of petroleum distillationproduct, having a density of 0.707 to 0.722 and a boiling range of 90120C.

about 3,250, polyoxyethylene making up approximately 10% of the totalweight of the molecule; this surfactant has an HLB value of about 1, isstrongly lipophilic, and functions as a wetting agent and defoamer.Pluronic P is a paste having a polyoxypropylene nucleus with an averagemolecular weight of about 2,250, polyoxyethylene making up approximately50% of the total weight of the molecule; this surfactant has an HLBvalue of approximately 16 and functions primarily as a detergent.Pluronic P-l04 is a paste having a polyoxypropylene nucleus with anaverage molecular weight of approximately 3,250, polyoxyethylene makingup approximately 40% of the total weight of the molecule; thissurfactant has an HLB value of approximately 13 and is particularlyuseful as an emulsifying agent.

The cleaning composition just described is extremely useful for a widerange of metal cleaning operations. Thus, although metal parts vary insurface area and amount of scale, and although such parts are often notcompletely drenched with oil, this composition is suitable for cleaningoperations in which the worst reasonably anticipated conditions might beencountered.

The novel one-shot method taught herein provides simultaneous mechanicaland chemical cleaning of metal parts which are to be subsequentlyovercoated. The parts are placed in an aqueous descaling and degreasingcomposition, which is then vigorously agitated while the parts are beingmechanically cleaned to loosen the scale and oil; after cleaning hasbeen completed, the parts are rinsed and then overcoated as desired. Thecleaning and rinsing operation is conveniently effected in an inclinedrotary open end barrel, the impingement of the parts on each other andthe walls of the barrel during rotation serving to loosen the scale, oiland dirt which contaminates the metal substrate. The mechanical cleaningis rendered more efficient by including small particles, or impactmedia, in the charge.

In a typical cleaning operation, 6 cubic feet of metal articles to becleaned are charged to an obliquely disposed ZOO-gallon octagonal rotarytreating barrel. To the barrel are then added 3 quart of the cleaningsolution of the example, 30 gallons of water, and 6 cubic feet of impactmedia (e.g., a blend of glass beads, having an average diameter of 0.012to 0.022 inch and 10% having a diameter of 0.006 to 0.007 inch). Livesteam is introduced to heat the charge to near 212 F. and the barrel setin motion. After approximately 15 minutes of operation the barrel isturned to rinse position and rotated while rinse water is suppliedcontinuously for 5 minutes at a rate of about 12 gallons per minute. Theexcess liquid is then decanted, and, if mechanical plating is to becarried on in the same barrel, a conventional anchoring solution (suchas a coppering solution) and a mechanical plating charge aresuccessively introduced. If, alternatively, the cleaned articles are tobe painted, electroplated, or otherwise overcoated, they are screenedfrom the impact media and transported to the next station.

In a test (described in SAE 1065-1080) for determining the hydrogenembrittling characteristics imparted by cleaning, a standard high carbonsteel spring wire hose clamp having a Rockwell C hardness of at least 53is cleaned, plated, and, after a given lapse of time, spread with astandard instrument and slipped over a bar which is slightly greater indiameter than the normal ID. of the clamp. If the hose clamp has beenhydrogen-embrittled, it will fail, i.e., break, either immediately, orwithin 24 hours; generally speaking, the shorter the time lapse beforetesting, the greater the probability of failure. A total of 150 No. 15hose clamps were cleaned as just described, given a copper anchor coat,and immediately mechanically plated with zinc. Four hours after plating,50 of the clamps were mounted on a test bar having a diameter of 0.953inch, no failures occurring within 24 hours. Twentyfour hours afterplating, the other clamps were mounted on the test bar; again nofailures occurred within 24 hours.

The example herein is, of course, intended to be merely illustrative,and numerous variations can be made in the practice of the invention.For instance, many acids-or blends of acidsmay be employed. Mineralacids such as sulfuric and phosphoric are strong and less expensive,offering obvious economic advantages. Because such mineral acids attackeven very stable surfactants in concentrated form, it is often preferredto add the acid and the remaining ingredients separately at the time ofcleaning. Generally, too, the stronger inorganic acids tend to dissolvethe metal at a rapid rate, and, particularly in the case of high carbonor heat-treated steel parts, a carbon smut residue often remain on thesurface. This condition may be alleviated by adding a small amount ofone or more soluble salts, e.g., sulfates, of tin, lead, cadmium,mercury, etc. It is also desirable under such conditions to make certainthat a carbon soil dispersant, i.e., a high- HLB surfactant, is presentin the surfactant mixture. Phosphoric acid, as is well known, offersparticular advantages when the surface to be cleaned is thereafter to bepainted. Likewise, it is convenient to use the acid salts of mineralacids (e.g., sodium bisulfate or monobasic sodium phosphate) or saltswhich hydrolyze to form available hydrogen ions (e.g., aluminum chlorideor ammonium sulfate).

Many other organic acids, both monobasic and multibasic. may beemployed. For example, tartaric, citric, diglycolic, chlorosulfonic,formic, acetic, and mono-, dior tri-chloroacetic acid may be employed.In calculating the available hydrogen ions which an acid can supply,only those acid radicals having a dissociation constant of at least l0are considered effective. Acids which might the considered too weak arerendered more effective by the preferred elevated cleaning temperature.

Glycolic acid possesses the advantage that it is a sequestering agentand hence aids in competing with the metal for the oxygen atoms whichare in the oxide coating; similarly, where the available water supplycontains an ion which interferes with cleaning action, glycolic acidaids in sequestering such ions. Where it is desired to provide asequestering agent while employing an acid which does not have thischaracteristic, other acid-stable sequestering agents may be added;thus, Versenex 80 (based on a diethylene triamide penta acetic acidadduct) or Clarotex may be used.

The Pluronics are attractive surfactants to work with because theirstability, chemical similarity, and physical characteristics have beendetermined. The surfactant mixture of the cleaning solution describedherein may, however, be any acid-stable materials of suitable HLB havingthe following ionic nature: all anionic, a blend of anionic andnon-ionic, all non-ionic, a blend of non-ionic and cationic, and allcationic. Such N,N-ethoxylated unctuous nitrogenous compounds asEthomeen C/ 25 (coco amine which is N,N-substituted with a total of 15mols of ethylene oxide to form a cationic surfactant) or Ethomid HT/ 25(hydrogenated tallow amide which is N,N-substituted with a total of 15mols of ethylene oxide to form a nonionic surfactant) tend to adsorb onmetal surfaces, reducing the need for low HLB wetting agents and aidingin the otherwise diflicult cleaning of metal articles having a thintightly adsorbed film, e.g., of a sulfonated oil. The addition of thesesurfactants thus extends the usefulness of the cleaning compositionsdescribed herein.

A number of materials have been empirically found to inhibit hydrogenembrittlement. Composition A, described in connection with the exampleherein, contains a combination of materials which are individuallyeffective in this regard, both the polyoxyethylene glycol and theo-toluidine being able to reduce or suppress hydrogen embrittlement,even at high temperatures. In Composition A some of these materials arereacted, resulting in an adduct of orthotoluidine and and formaldehydehaving the characteristics of stability and excellent inhibition. As amatter of practicality, however, a wide variety of commerciallyavailable inhibitors, such as Polyrad 1110A, Armohib 28, or hexynol,substituted thioureas, and most amines or proteins, can be employed withat least some degree of success. Inhibitors are at least somewhatselective, however, and a given inhibitor may be more effective with oneacid than with another.

What We claim is:

1. A balanced composition for preparing a wide variety of scaly, oily,dirty, or otherwise contaminated ferrous metal surfaces for subsequentovercoating, consisting essentially of a water-soluble mixture of:

sufiicient strong acid, having a dissociation constant greater than 10-to provide 1-5 parts by weight of hydrogen ion, and

an acid-stable surfactant system consisting essentially of substantiallyequal parts by weight of the following individually acid-stablecomponents:

dispersant having an HLB value of at least 17, detergent having an HLBvalue in the range of 12-17, sufiicient emulsifier to emulsify parts byweight of mineral spirits, said emulsifier having a higher molecularweight than said detergent and an HLB value in the range of 1217, andwetting agent for ferrous metals, the lipophilic groups being dominantin said wetting agent, having an HLB value on the order of 5 or less.

2. The composition of claim 1 wherein the wetting agent is a surfactantselected from the class consisting of coco amine N,N-substituted withethylene oxide and bydrogenated tallow amine N,N-substituted withethylene oxide.

3. The composition of claim 1 wherein each of the components of thesurfactant system has a hydrophobic polyoxypropylene nucleus withhydrophilic polyoxyethylene chains graft-polymerized thereon.

4. A one-shot method of preparing ferrous metal articles for subsequentovercoating comprising: (1) placing in a rotary container, impact media,water, on the order of 10,000 parts by weight of small ferrous articlesto be cleaned, and the composition of claim 1, (2) rotating saidcontainer to both mechanically and chemically clean, descale, and desmutthe metal articles, and (3) removing the cleaning solution.

5. The method of claim 4 in which live steam is supplied to heat thecontents of the container.

References Cited UNITED STATES PATENTS 2,261,700 11/1941 Ryznar 252143 X2,326,837 8/1943 Coleman 2,52-142 2,399,267 4/1946 Szatyn 134-282,571,581 10/1951 Kearney 13433 X 2,683,343 7/1954 Gellette et al. l34-7X 2,942,956 6/1960 Kelly 1343 X 3,041,285 6/1962 Newman 134-3 X3,162,547 12/1964 Kendall 134-41 X LEON D. ROSDOL, Primary Examiner.

ALBERT T. MEYERS, Examiner.

M. WEINBLATT, Assistant Examiner.

1. A BALANCED COMPOSITION FOR PREPARING A WIDE VARIETY OF SCALY, OILY,DIRTY, OR OTHERWISE CONTAMINATED FERROUS METAL SURFACES FOR SUBSEQUENTOVERCOATNG, CONSISTING ESSENTIALLY OF WATER-SOLUBLE MIXTURE OF:SUFFICIENT STRONG ACID, HAVING A DISSOCIATION CONSTANT GREATER THAN10**-6, TO PROVIDE 1-5 PARTS BY WEIGHT OF HYDROGEN ION, AND ANACID-STABLE SURFACTANT SYSTEM CONSISTING ESSENTIALLY OF SUBSTANTIALLYEQUAL PARTS BY WEIGHTS OF THE FOLLOWING INDIVIDUALLY ACID-STABLECOMPONENTS: DISPERSANT HAVING AN HLB VALUE OF AT LEAST 17, DETERGENTHAVING AN HLB VALUE IN THE RANGE OF 12-17, SUFFICIENT EMULSIFIER TOEMULSIFY 150 PARTS BY WEIGHT OF MINERAL SPIRITS, SAID EMULSIFIER HAVINGA HIGHER MOLECULAR WEIGHT THAN SAID DETERGENT AND AN HLB VALUE IN THERANGE OF 12-17, AND WETTING AGENT FOR FERROUS METALS, THE LIPOPHILICGROUPS BEING DOMINANT IN SAID WETTING AGENT, HAVING AN HLB VALUE ON THEORDER OF 5 OR LESS.